The ready availability of integrated circuits has made possible the manufacture of extremely compact broadband radios. The widespread use and availability of such compact radios has created a need for physically small broadband antennas, especially where very high frequency (VHF) transmission and reception is involved.
The military have a particular and special need for smaller size broadband antennas than are generally available for VHF applications.
Size reduction of broadband antennas has not been fully realized because of the dilemma posed by the dictates of the size relationship between fundamental antenna gain and bandwidth limitations. Smaller broadband antennas experience reduced gain and narrowed instantaneous bandwidth unless they are equipped with complicated tuning devices which of necessity increases their size.
The advent of spread-spectrum radios, such as Sincgars, has further extended the need for low profile rugged antennas for military use in portable radio equipment (manpack) as well as vehicular radio communications. This type of VHF communication must be capable of operating in a frequency-hopping mode over a 3 to 1 frequency range. Another drawback of current broadband antenna designs involves the mounting on vehicles which could adversely modify the antenna impedance from its normal value because of the electrical disturbances due to the excitation of currents on the vehicle. This also causes a change in the shape of the radiation pattern such as undesirable ripples as opposed to desired uniform patterns.
One of the important considerations in the design of an antenna is the maintenance of an appropriate input impedance to the broadband antennas of the type to which this invention is directed. It is most important to the transfer of power from the transmitter to the antenna or from an antenna to a receiver. To maximize the power transfer between the antenna and a transmitter or receiver, the antenna impedance should be a conjugate match. A conjugate match defines equal resistances and equal magnitude and oppositely signed reactances. Typically, the receiver (or transmitter) has a real impedance so it is necessary to negate ("tune out") the antenna reactance with a matching network of variable inductances and capacitances to cancel antenna reactances.
Designers of antennas have experienced disadvantages with antenna impedance matching networks. It has been found that such matching networks are inefficient because of ohmic losses in tuning coils and the matching network has been found to perform over a relatively narrow band of frequencies. This problem of course reduces the operational bandwidth, which is an undesirable result. The problems resulting from a mismatch at the antenna results in setting up reflections along the transmission line causing the voltage standing wave ratio ("VSWR") to be much greater than unity. The VSWR defines the range of frequencies for which performance of the antenna conforms to a specific standard. For example, the VSWR desirably is less than 3 to 1.
Certain prior art patents involving the use of matching networks controlling the impedance in broadband vertical antennas deserve discussion. Such innovation is disclosed in two U.S. patents, U.S. Pat. No. 2,913,722 and U.S. Pat. No. 3,100,893, both issued to Helmut Brueckmann. Brueckmann teaches the use of isolation chokes which may or may not use ferrite cores. Brueckmann's chokes are adjustable in stepped increments by switching a tapped shunt inductance coil. The incremental step adjustments are based on the node position of the current distribution along the antenna. To improve the impedance match this prior art varied the node position of the current distribution. These adjustments degraded the effectiveness of the base-isolation feature. These prior art antennas being dual-feed type require that the feed point be at the base of the antenna for low frequencies. The feed point is switched to the center feed for higher frequencies. In order to cover a broadband, the teaching of these prior art patents requires switching at least 10 sub-bands to achieve the range of 20 to 70 MHz.
It is therefore a primary object of the present invention to provide a broadband VHF antenna that is smaller in size and of rugged construction and provides a low VSWR over a 3 to 1 frequency range.
It is another object of this invention to provide a broadband VHF antenna that can be mounted on fixed or mobile support platforms with the antenna having reduced physical height, improved input impedance, increased effective electrical height and providing a low VSWR over a broad frequency range.